when you downshift youre using a lower gear to either go faster slower. the engine at a specific rpm is spinning the driveshaft at that particular speed. the transmission translates the speed from the driveshaft into rpms at the wheels. therefore you can think of it like shifting up through the gears in a 21 speed bike. however the same comparison cannot be made when downshifting. the engines rpm is controlled by the amount of gas the driver is giving the engine. thus someone can lower the gear ratio without using the gas pedal and decelerate the car by letting the engine brake itself because the rpms at the wheel are greater than the rpms at the driveshaft. basically the pressure is taken off of the brakes and onto the transmission.

i hope this helps and i think its accurate...someone correct me if im wrong.

chet